An oil separator described in the specification of European Patent (EP) No. 1062162, for example, is known as this conventional type of oil separator. This oil separator includes a wall portion that is at a downstream side of a circular shaped elastic tongue piece (flap), attached to a flow path of the blow-by gas in a cantilevered state, and that runs along an outer periphery of the elastic tongue piece. Oil mist contained in the blow-by gas can be captured by a slight loss in pressure due to the blow-by gas that has pressed open and flowed past the elastic tongue piece striking the wall portion.